1. Field of the Invention
This invention relates to a braking device for a focal plane shutter for a camera.
2. Description of the Prior Art
The prior known braking device for shutters is made to brake the shutter curtain within a relatively short stroke near the terminal end of running movement of the shutter, for the curtain is prevented from bouncing. But when the frictional force is too strong at the start position of braking, the shutter curtain adversely bounces, producing disadvantages that the exposure becomes uneven near the side of the picture frame and that the curtain stops without having to reach the terminal end of running movement.
For this reason, a proposal has been made for such a braking device that the force of a compression spring for giving a braking force to the shutter is relatively weak at the braking start position and gradually increases to insure that the shutter is stopped at the exposure aperture fully closing position, as, for example, disclosed in U.S. Pat. No. 2,218,767.
In case when another brake which cannot change the pushing pressure of the spring for exerting frictional forces between the initiation and termination of a braking operation because of no availability of space, since, from the nature of friction, the static friction is larger than the dynamic one, it results that the braking force is maximum at the braking start position and then gradually increase. Therefore, for an adequate braking force is obtained in the closing position, the frictional force to take action in the early stage becomes very strong, thus giving a shock to the shutter curtain, reducing the durability, and producing the aforesaid phenomenon that the exposure is made out of uniformity, and the full closing of the exposure aperture fails. Such static friction tends to increase particularly at low temperatures.
Also in the blade type focal plane shutter, as the blade is deformed by the shock in the braking start position, the border of the trailing curtain collides against the edge of an exposure aperture in a partition plate intervening between the leading and trailing curtains and is hindered to run down. There will be also a high possiblity of damaging the curtain.